The invention relates to an apparatus for generating dense plasmas for vacuum processes using alternating polarity pulsed hollow cathode arc discharge sources.
It is known that in the low-pressure range between 10−2 Pa and 1 Pa very high charge carrier densities on the order of magnitude of 1012 cm−3 can be achieved with hollow cathode arc discharge plasmas. If, during layer deposition, the layer-forming particles are exposed to such a plasma, advantageous coating properties can be achieved.
For this purpose, apparatuses are known in which the vapor can be activated effectively by using the hollow cathode plasma exclusively for the plasma activation, in particular for larger coating areas and high coating rates. The plasma discharges of these apparatuses are operated using direct current. In such apparatuses, the anodes required in addition to the hollow cathodes for plasma generation are partially connected to the vaporization device (DE 196 12 344 C1). The utilization of the vaporization crucible or of an electrode arranged as an anode close to the vaporization crucible has the disadvantage that the functional capability of the apparatuses is limited to a plasma activation associated with the vaporization of electrically conducting materials.
Moreover, it is known to guide the plasma by means of a magnetic field. If appropriate magnetic field-generating devices generate a longitudinal magnetic field so that a number of its field lines lead from the hollow cathode to the anode, higher-energy beam electrons on their way from the hollow cathode to the anode remain bound in the range of the connecting field lines and they keep the high-density plasma a certain distance from the substrate (DE 42 35 199 C1). A disadvantage here is that, by keeping the higher-energy beam electrons away from the substrate, the buildup of a high self-bias potential is prevented. An additional disadvantage is that the usability remains limited to special designs of the vaporization device with expensive magnetic systems. In addition, in the case of vaporization of insulating materials, a high apparatus cost is required in order to maintain the electrical conductivity at the anode.
DE 195 46 827 A1 describes an apparatus for the vaporization of insulating materials, in which a ring-shaped anode (a so-called ring anode) is arranged directly before the hollow cathode in order to generate effective plasma for the plasma activation exclusively by the beam electrons that have passed through the opening of the ring anode. In this case, no interfering insulating layers are deposited on the ring anode, because the latter is located substantially outside of the vapor region and the ring anode is strongly heated as a result of the impinging beam electrons. However, since the area of the plasma activation of the vapor is located outside of the electrical field built up between the hollow cathode and the anode, there is the disadvantage that only clearly lower charge carrier densities in the plasma can be generated than with arrangements that enclose the area to be activated between the hollow cathode and the anode. In addition, the range of the beam electrons delimits the geometric extent of the plasma.
In DE 199 02 146 A1, it is proposed to arrange two plasma-generating devices known from the aforementioned patent application, each including a hollow cathode with associated ring anode and a direct current voltage source connected in between, so that they are located opposite each other and generate a bipolar, medium-frequency pulsed voltage between the two hollow cathodes. In this arrangement, due to the direct current voltage source between the hollow cathode and the ring anode, there is always plasma generated between the hollow cathode and the associated ring anode of a plasma-generating device. In the phases in which one of the two plasma-generating devices is connected cathodically due to the bipolar voltage pulse, the plasma of the cathodically connected plasma-generating device is drawn by its hollow cathode to the ring anode of the opposite plasma-generating device. In this manner, a large-volume hollow cathode arc discharge plasma forms between the two ring anodes. The disadvantage of this technically demanding construction is that numerous process parameters have to be adjusted to achieve constant plasma conditions.